This invention relates to methods of forming a screw thread on a crankshaft and the like, and more particularly it deals with a method of performing roll threading on a portion of a crankshaft and the like which is subjected to case hardening.
To produce a crankshaft for use with a small engine, for example, it is necessary to subject the material for producing the crankshaft to case hardening. When the operation of forming a screw thread on the crankshaft is performed, it has hitherto been usual practice to use one of the following two methods. One method consists in first forming a screw thread on one portion of the crankshaft either by machining or by roll threading before carburizing is performed, and then performing carburizing on other portions of the crankshaft than the threaded portion which is prevented from being subjected to carburizing, and the other method comprises the steps of subjecting the whole body of the crankshaft to carburizing after a screw thread is formed beforehand, and tempering only the threaded portion by suitable means.
These methods of the prior art have suffered the disadvantage, particularly when roll threading is relied on for forming a screw thread, that the screw thread formed fails to meet the required standards of precision because of inner stresses developed in the threaded portion of the crankshaft, as a result of the carburizing being performed following the roll threading. To ensure that the screw thread formed meets the required standards of precision, it is necessary to provide another process step, thereby rendering the operation troublesome and time-consuming. In addition, the methods of the prior art described hereinabove require means for preventing the threaded portion from being subjected to carburizing or means for tempering the threaded portion, thereby increasing cost.